1. Field of the Invention
The present invention relates to an encoding/decoding circuit which ensures the safety of data requiring concealment.
2. Description of the Background Art Patent Document 1, Japanese Patent Application Laid Open Gazette No 2004-88505, discloses an encoding/decoding circuit which can avoid complication of key selection for stream data inputted in parallel. In this circuit, to a plurality of input interfaces, stream data of TV broadcast, stream data recorded in a DVD, stream data of CATV line or the like are inputted in parallel (see Paragraph 0015), and its arbitration portion outputs a decoding key and input data in accordance with an input channel to an encoding/decoding operation portion (see Paragraph 0038). Then, the encoding/decoding operation portion decodes the encoded data which is inputted by using the decoding key inputted from the arbitration portion and outputs the decoded data (see Paragraph 0039).
In Patent Document 1, the construction to set the content of key data in registers 131 to 134 shown in its FIG. 1 is only described as “keys 131 to 134 are each formed of a register and a decoding key of a program in accordance with channel selection of a viewer is set by a control portion of a TV receiving device” (see Paragraph 0025), but no specific description is made on a specific circuit configuration to set the content of the key data. Therefore, in view of this description, it is thought that a plurality of wires are simply provided to supply key data from an input interface or an input stream processing portion to the respective registers 131 to 134. Providing the wires from the input interface or the input stream processing portion to the respective registers 131 to 134, however, causes an increase in the number of wires and complication of circuit layout.
Further, in Patent Document 1, a key storage portion of its FIG. 1 only includes one register (each of 131 to 134) for a channel of data to be encoded or decoded. In order to generate a new key from stream data by using a key stored in the register, it is necessary to obtain the second key through decoding by using the first key stored in the register and then write the second key over the register. After that, in order to obtain a key other than the second key through decoding by using the first key, it is necessary to write the first key over the register again. Such an encoding/decoding operation disadvantageously causes a decrease in throughput of data processing.
Furthermore, Patent Document 1 has a problem that information of a key which requires concealment and output stream data which requires concealment are easily acquired from the outside in an illegal manner and easily interpolated.
If a block encoding/decoding system, such as a CBC (Cipher Block Chaining) system or a CFB (Cipher FeedBack) system, is adopted, it is necessary to give an IV (Initialization Vector) for generation of the first key. A key generated from this IV may be stored inside a chip in which an encoding/decoding circuit is formed or in a nonvolatile memory or the like, which is provided outside.
If a refined process is adopted to reduce the chip cost, however, it becomes difficult to integrate the nonvolatile memory for storing a key and an encoding/decoding program in one chip in which the encoding/decoding circuit is formed. This is because it is difficult to refine a nonvolatile memory and if such a leading-edge process as realizes a gate length of 90 nm is adopted, an encoding/decoding circuit can be formed while it is difficult to form a nonvolatile memory in the same chip through the same process.
It is a matter of course that the safety of key should increase if a nonvolatile memory is integrated in a chip in which an encoding/decoding circuit is formed. The reason is that if a nonvolatile memory is provided outside and the nonvolatile memory which is a general-purpose product is made removable, it becomes easy to remove the nonvolatile memory and interpolate the content of a key stored therein.
In other words, the background-art encoding/decoding circuit of Patent Document 1 gives no consideration to the security of information of a key which requires concealment.